In numerous industrial and commercial processes, it may be desirable to provide high purity germane. One area in which high purity germane is typically required is in the fabrication of semiconductor devices such as transistors, diodes, integrated circuits, detectors, solar cells, and the like. In many of these applications, high purity germane is oftentimes used as a gas for doping substrates. More recently, the commercial use of germanium tetrahydride by the semiconductor and solar cell manufacturers has steadily increased because of new technology that incorporates germanium into active silicon structures. This new technology requires that germane be produced at higher purity levels with less variability in impurity concentrations and a lower cost than present manufacturing methods.
There are many known methods for the production and synthesis of germane. Typical synthesis methods may be categorized as a chemical reduction method, an electrochemical reduction method, or a plasma synthesis method. The chemical reduction method typically involves contacting a germanium-containing compound such as germanium tetrachloride, germanium oxide, a germanide, and/or elemental germanium with a reducing agent such as, but not limited to, sodium borohydride, potassium borohydride, lithium borohydride, lithium aluminum hydride, sodium aluminum hydride, lithium hydride, sodium hydride, or magnesium hydride. In one common chemical reduction method, a germanium-containing compound is added to an aqueous acid solution which is then subsequently added to an aqueous solution containing the reducing agent sodium borohydride to conduct the chemical reduction reaction. Other chemical reduction methods may use an organic solvent solution rather than an aqueous solution as the reaction medium or may use the reaction of germanides with acid. The electrochemical reduction method, on the other hand, typically involves applying voltage to a germanium metal cathode that is immersed in an aqueous electrolyte solution and an anode counter-electrode composed of a metal such as molybdenum or cadmium. In this method, germane and hydrogen gases evolve from the cathode while the anode reacts to form solid molybdenum or cadmium oxides. Yet another electrochemical reduction method may involve applying voltage between a cathode, comprised of a standard material such as nickel or tin, and an anode of standard material that are immersed in an aqueous electrolyte solution containing a germanium-containing compound to produce a germanium and hydrogen-containing gaseous product evolving at the cathode. Lastly, the plasma synthesis method involves bombarding elemental germanium with hydrogen atoms (H) that are generated using a high frequency plasma source to provide various reaction products such as GeH4 and Ge2H6.
Typical synthesis methods traditionally produce germane that may be contaminated with variable amounts of germanium-containing impurities such as, but not limited to, higher order germane compounds (i.e., digermane, trigermane, etc.), chlorogermanes, and germoxanes. Further, germane can slowly decompose during storage to generate variable amounts of these germanium-containing impurities. These germanium-containing impurities may make the germane unusable for its end-use application due to the negative impact on deposited film properties and on process control and processing. In this connection, higher amounts of the germanium-containing impurities may effect the incorporation of germanium into active silicon structures and generate unacceptable levels of defects in the structures.
There is a need in the art for a commercial scale, cost-effective process for the synthesis and/or purification of germane that gives higher purity germane and less variability in impurity concentrations.